This invention relates to an organic gel of decontamination, usable for radioactive decontamination of surfaces, especially metallic surfaces.
According to the invention, xe2x80x9corganicxe2x80x9d gel is a gel in which thickening agents are essentially organic, in other words, excluding every inorganic or mineral substance.
Decontamination of parts tarnished by radioactive elements can be made either by mechanical treatments or by chemical ones.
The methods using mechanical treatments are disadvantageous in that they entail a more or less significant modification of the part""s surface. Another disadvantage is that these treatments are difficult to apply on parts of complicated shape.
The methods of treatment by soaking essentially consist of carrying away radioactive elements fixed on the surface of the part by means of solution of actual appropriate agents of decontamination, especially of Ce(IV) stabilized in acid environment, highly concentrated such as nitric acid or sulfuric acid. These methods present the disadvantage of production of a large volume of effluents whose ulterior treatment, particularly by concentration, is very onerous.
Besides, the methods of treatment by soaking implementing solutions raise some problems in the treatment of pieces of large dimensions. It is very difficult to immerse and totally soak these pieces in the solution of reactors.
The solutions of decontamination, in effect, allow the treatment by soaking only for metallic disassembling pieces of limited sizes, in other words, in practice, these solutions can be practically utilized only in dismantling of radioactive installations.
On the other hand, the decontamination on the spot of radioactive installations, by spraying of watery solutions, produces large quantities of active effluents for a limited efficiency, because of a short-term contact with the pieces.
Thus, it has been suggested to transform into a viscose form the decontaminating solutions that include an active agent, which is formed by viscose/gelifying agents, in particular by solids divided in great specific areas, in small sizes of elementary particles and chemically inert.
Among the solids which comply with these requirements, supports of minerals, such as aluminum and silica, readily available in the market, present moreover a large diversity of their characteristics such as a hydrophilic or a hydrophobic character, pH, etc. and appear to be the best way to transform these solutions to viscose/gel form.
The spraying of such gels, unlike the solutions, can allow the decontamination on the spot of important metallic surfaces, which are not necessarily horizontal, but can be inclined or even vertical.
The decontaminating gels can be described as colloidal solutions including a thickening agent generally mineral, such as alumina or silica, and an active agent of decontamination, i.e. an acid, a base, an oxidizing agent, a reducing agent or a mixture thereof, which is chosen in function of the nature of the decontamination and the surface.
Thus, an alkaline gel for stainless steels and ferritic steels, will present the de-greasing properties for the elimination of the non-fixed contamination.
An oxidizing gel for stainless steels will perform the elimination of the heat or cold fixed contamination. A reducing gel will preferably be used in complement of the oxidizing gel and in an alternate way for the dissolving of oxides, heat-formed i.e. in the primary circuit of pressurized-water reactor.
Lastly, an acid gel for ferritic steels will perform the elimination of the contamination fixed in cold.
The utilization of gels for the radioactive decontamination of pieces is described particularly in document FR-A-2 380 624.
In this document, a decontaminating gel consisting of a colloidal solution of an organic or mineral compound is used in which one possibly adds a decontaminating product such as hydrochloric acid, stannous Chloride, oxine and/or sodium fluoride.
Though these gels give satisfying results, they sometime present the disadvantage of not being able to eliminate incrusted radioactivity only on a reduced depth of the part""s surface, e.g. on a depth of about 1 xcexcm.
Document FR-A-2 656 949 describes a decontaminating oxidizing gel which allows to eliminate radioactive elements deposited on the part, as well as radioactive elements incrusted on its surface.
This decontaminating gel is consists in a colloidal solution that includes:
a) from 8 to 25% by weight of a mineral-gelifying agent, preferably in silica, pyrogened silica or aluminum base;
b) from 3 to 10 mol/l of mineral base or of a mineral acid.
c) from 0.1 to 1 mol/l of an oxidizing agent such as CeIV, CoIII, or AgII, that has a normal oxide-reduction potential E0 higher than 1400 mV/EVH (standard hydrogen electrode) in a strong acid environment or in the reduced form of this oxidizing agent.
In the last case, the gel also includes from 0.1 to 1 mol/l of a compound (d), which is able to oxidize the reduced form of this oxidizing agent.
In the decontaminating gel described above, the presence of components (b) and (c) assures the elimination of radioactive accumulations, formed on the piece""s surface as well as the elimination of incrusted radioactivity, by controlled erosion of the surface that has to be decontaminated.
However, this oxidizing gel does not present a sufficient effectiveness in relation to layers of adhesive metallic oxides, deposited on the surface of the alloy such as austenitic steels, Inconel 600 and Incoloy.
Document FR-A-2 695 839 describes, a reducing decontaminating gel which allows to eliminate these layers of adhesive metallic oxides, and which includes:
a) from 20 to 30% by weight of a mineral-gelifying agent, preferably of alumina base.
b) from 0.1 to 14 mol/l of a mineral base, such as NaOH or KOH, and
c) from 0.1 to 4.5 mol/l of a reducing agent, that has a potential of oxide-reduction, less than xe2x88x92600 mV/ENH in an environment of a strong base (pHxe2x89xa713), chosen among the borohydrides, sulphites, hydrosulphites, sulfides, hypophosphites, zinc and hydrazine.
The application of gels on the surface, for example the metallic surface, is preferably accomplished by spraying, for example under a pressure that could range from 50 to 160 bars and beyond, the gel being agitated before the spraying to transform it into a homogenous gel. After a proper duration of action, the gel is rinsed by spraying water, then the effluents generated are treated by neutralization, decantation and filtration.
All the gels described above, whether they be alkaline or acid, reducing, or oxidizing, present the advantages already mentioned such as the possibility to treat the pieces of complicated shape, an easy application, a small quantity of sprayed chemical radioactive elements by specific area, i.e. a small quantity of effluents, produced by rinsing the applied gels and a perfectly controlled time of contact with the surface, hence a control of the erosion during decontamination. Moreover, the fact that it is possible to spray the gel in a distance significantly diminishes the amounts absorbed by the agents meant for the radioactive purification.
The typical gels of prior art are marketed by xe2x80x9cF.E.V.D.I.xe2x80x9d company, under the name of xe2x80x9cFEVDIRADxe2x80x9d.
All the gels mentioned above, whether they be alkaline, reducing and especially oxidizing, present a corrosive power.
Unfortunately, they cannot stand the high shearing velocities imposed by spraying, which is the most traditional process for the application of gels.
In fact, all these gels including a mineral thickening agent, particularly silica, whether it be hydrophilic, hydrophobic, basic, or acid, have rheological properties, characterized by a thixotropic behavior; during the spraying, the viscosity diminishes under shearing, while after the cessation of shearing, the resetting process of the adhering gel on the surface starts. Such a fluid is characterized by a rheogram in hysteresis.
The control of this thixotropy is fundamental to allow a spraying and an optimal adherence of the gel onto the surface that has to be treated. The speed of the resetting process of the gel, either partial or total, is the most important concept for the spraying of gels.
Indeed, the resetting process means a return to the gelification, hence an adherence on the surface and a short resetting time characterizes a gel, which quickly recuperates a sufficient viscosity after spraying to prevent any sagging.
Whatever the charge in mineral thickening agent of the gels described above or currently marketed, the resetting times are too long. For example, for various charges in Cab-o-Sil(copyright) M5, which is a hydrophilic pyrogen silica and acid marketed by xe2x80x9cDEGUSSAxe2x80x9d company, the resetting times are always longer than 5 seconds, which is very excessive.
The period of return to a sufficient viscosity, in order for the gel to adhere on the surface, can be very short, but this requires a significant increase of the mineral charge.
Before the spraying, the viscosity under agitation is high and the process of spraying becomes difficult. Furthermore, this increasing mineral charge produces large quantities of rinsing effluents and solid waste for treatment.
For example, after the treatment by filtration of the rinsing effluents, 20 kg of gel give a volume of radioactive waste corresponding to a barrel of 200 liters.
There is a chance to improve the rheological properties of the gels described above including a thickening agent, exclusively mineral, consists in micrometric particles of smoked silica or by a mixture of alumina and silica. The rheological properties of these gels can be improved by reducing their mineral charge and in the same time, without affecting their corrosive qualities.
Such improvements have been obtained in document FR-A-2 746 328 which describes an organic mineral gel of decontamination consisting of a colloidal solution that includes a thickening agent and an active agent of decontamination. The thickening agent includes the combination of a mineral thickening agent, such as silica or aluminum, and an organic thickening agent or co-thickening chosen among the water-soluble organic polymers, such as the polymer of the acrylic acid and its copolymers with acrylamide.
According to this document, the incorporation of decontaminating gel in the viscose gel, in addition to a mineral thickening agent (called co-thickening), allows to improve Theological properties of gels, and to significantly diminish their mineral charge and the produced solid waste without affecting corrosive properties and their qualities of decontamination.
The addition of the organic co-thickening agent leads to a diminution of the mineral""s charge, which in this case is about 5% by weight, instead of 20% by weight, for the previous gels. This fact appeals the thickening agents and especially mineral ones.
The gels described in this document can be perfectly projected, easy to eliminate by rinsing after application. In addition, they present an easy filtration at the time of the treatment of the effluents and a diminished volume of the final solid waste as well.
Besides, the polymer or surfactant organic co-thickening agent is easy to deteriorate at the time of the treatment of the effluents.
However, though diminished to approximately 5%, the mineral charge of gels described in document FR-A-2 746 326 is still important, since it is generally about 5%, which involves the need for a more complex system of filtration.
In addition, the personnel who intervene to recuperate this residual mineral charge is exposed to a certain dose of radiation.
Besides, it is important to reduce in a general way, the volume of solid waste, produced during the process of decontamination, considering the problems of storage and protection of environment.
Therefore, there is a need for a decontaminating gel whose mineral charge, which comes from the thickening agent, is quite diminished, even substantially cancelled, in order to generate a minimal volume of solid waste. This diminution or suppression of the mineral charge should be obtained without affecting the other properties of the gel.
These other properties, that should not be affected, are particularly rheological properties. The resetting time should be the shortest possible and the system should be sufficiently liquid under agitation, to allow one spraying.
In addition, the corrosive qualities of these gels should not be deteriorated and the obtained decontamination factors should be at least as identical as those of existing gels.
The object of this invention is to provide a decontaminating gel, which satisfies all needs mentioned above.
Another object is to provide a decontaminating gel, which does not present the disadvantages, limitations and inconveniences of the methods of prior art and which resolves all the problems of prior art.
This goal and others are reached, according to the invention, by a decontaminating gel, which consists of a solution that includes:
a) a thickening agent;
b) an active agent of decontamination;
in which the thickening agent (a) is a thickening agent exclusively organic chosen among the water-soluble organic polymers.
According to the invention, the gels do not contain any viscose minerals such as silica or alumina. Consequently, because of the mineral charge, which is null, all the inconveniences due to solid waste created by this mineral discharge are eliminated, particularly a system of filtration and expensive and complex recuperation of these waste is no longer necessary.
The waste produced in small quantities contains only organic products, which can be easily deteriorated, such as compound of carbon, nitrogen, oxygen and hydrogen, without prohibited elements in the nuclear such as sulphur or halogen.
It should be noted that this total suppression of every mineral thickening agent is obtained without affecting the other fundamental properties in a negative way. The fundamental properties are especially Theological and corrosive properties as well. Particularly, the factors of decontamination obtained with the gels of the invention are completely comparable, even superior to analog ones of the prior art, which means the gels include the same agent of decontamination, but they consist of mineral thickening agent alone or in combination with a viscose, like in document FR-A-2 746 328.
Surprisingly, the gels according to the invention, keep their characteristic structure much longer than gels, which include a mineral thickening agent, and dry much faster, while preserving their properties of corrosion. Their elimination by rinsing is thus made easier and the volume of rinsing effluents diminishes.
Moreover, the gels in this invention are quite resistant under temperature i.e. up to 80xc2x0 C. or in other words an excellent resistance to heat, i.e. the recycling and the corrosion properties of these gels are, preserved among others, at high temperatures, This characteristic is especially important in certain specific utilizations where surfaces to be treated are permanently under high temperatures i.e. greater than or equal to 40xc2x0 C.
The preparation of the gel according to the invention is easy and fast and it appeals only to reagents, which are easily available and are cheap; so, the gels according to the invention can be implemented on a large scale and on an industrial plan.
The gels according to the invention are the result of a surprising undertaking, contrary to what could have been expected. In fact, nothing would make one expect that total suppression of the thickening mineral in gels of the prior art presented in FR-A-2 746 328 would lead to gels with all required features, especially regarding their rheology.
So it is evident that if we increase the quantity of thickening organic agent in the gel compositions of document FR-A-2 746 328, in order to compensate a diminution of the mineral charge (carried out to reduce the mass of waste), the obtained viscosity was not sufficient to contemplate a use by spraying and even a liquid was obtained if the concentration of the co-thickening agent increases.
Unexpectedly, according to the invention, totally concealing the mineral thickening agent does not bring the expected effects such as degradation of gel properties, but in contrary, leads to a gel with all required characteristics and even more.
The invention triumphs over a presumption and resolves the problems of the prior art.
According to essential characteristic of the invention, the thickening agent (a) is a thickening agent exclusively organic, which is chosen among the water-soluble polymers.
These polymers can be used in the gel in a percentage generally ranging from 1 to 11%, preferably 2 to 8% by weight or 4 to 6% by weight; in these amounts, they allow especially a significant improvement of rheological properties of gels and a total suppression of the mineral charge i.e. in alumina and/or in silica. The polymer has generally a molar mass defined by the average molar mass by weight from 200,000 to 5,000,000 g/mol. By polymer, according to the invention, one understands the homopolymers as well as copolymers, sequence or statistic.
Preferably, this polymer should meet a number of conditions related, in particular, to its utilization in the nuclear facilities.
First, it should be soluble.
It should not contain either sulphur or halogen, such as fluorine or chlorine prohibited in the nuclear domain, it should participate minimally in the global organic charge, it should be resistant in presence of the active agents of decontamination (b); for example a good resistance in acid and/or oxidizing environment or even in basic and/or reducing environment. Moreover, it should be little sensitive to the ionic force of environment and it should be thermally stable at the temperatures generally from 0 to 50xc2x0 C. and beyond.
Among the numerous water-soluble organic polymers, it has been evident that the polymer of acrylic acid and its copolymers with acrylamide fulfilled these criteria and allowed the preparation of a gel meeting the requirements mentioned above.
The polyacrylic acid polymer is consists in the repetition of the monomeric unit (I) as follows: xe2x80x94CH2CH(CO2H)xe2x80x94. The average molar mass by weight of the polyacrylic acid polymer is from 450,000 to 4,000,000. The average molar mass by weight is preferably 4,000,000. In fact, it has been obvious that the formation of one gel needs increasing percentages of polymer, with the diminution of the macromolecular chain. This is due to the fact that an important average molar mass by weight corresponding to longer length of chain should favor a better reticulation and so, the formation of a more viscous gel for a smaller quantity of polymer.
The copolymers of acrylic acid with acrylamide have an average molar mass by weight ranging from 200,000 to 5,000,000, preferably from to 200,000 to 4,000,000.
The percentage of each of the monomers in the copolymer of acrylic acid and acrylamide is variable; the copolymer will generally include from 95 to 60% by weight of acrylic acid and from 5 to 40% by weight of acrylamide.
A preferred copolymer is a copolymer of an average molar mass by weight of 200,000 and whose percentage by weight of acrylamide is 10%.
These copolymers can be block or random copolymers. The random copolymer, of formula (I) 
is thus structured from two types of blocks of variable lengths. One of the blocks is consists in acrylic acid monomeric units and the other one for acrylamide monomeric units.
Examples of suitable acrylic acid-acrylamide copolymers are the copolymers marketed by the SCOTT BADER(copyright) company, under the name of TEXIPOL(copyright), such as TEXIPOL(copyright) 63-510. This product is presented in the form of a watery solution of 25% of a polyacrylic acid-acrylamide copolymer (molar mass: 106; percentage of acrylamide is from 20 to 30%) dispersed in an organic phase composed of white spirit of toluene or isopar of 20% in the form of emulsion with 5% surfactant.
Possibly, the gels according to the invention can also include an organic surfactant that is included in the organic thickening agent.
The surfactants of the family of polyoxyethylenic ether of formula (II):
CH3xe2x80x94(CH2)nxe2x88x921xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2)nxe2x80x94OHxe2x80x83xe2x80x83(II)
called CnEm, meet the required criteria i.e., among others, a sufficient stability, especially in very acid, very oxidizing and electrolytically high media, such as decontaminating gels.
In the above formula, n defines the length of the aliphatic chain and is an integer that can vary from 6 to 18, preferably from 6 to 12; m defines the size of the polar head and is an integer that can vary from 1 to 23, preferably from 2 to 6.
Among these surfactants, the preferred compounds are: C6E2 (hexyl of di(ethylene glycol) ether), C10E3 and C12E4.
Such compounds CnEm are available from ALDRICH and SEPPIC(copyright) companies.
The nature of the surfactant depends on the type of the decontaminating gel implemented, i.e. the nature and the content of the active agent of decontamination (b) and the nature and the content of the polymer organic thickening agent.
Thus, the compounds CnEm are particularly adapted to be used in the gels that include polyacrylic acid and especially in the acid oxidizing gels that include polyacrylic acid.
In addition, the content in surfactant depends on the nature of the decontaminating gel and on the concentration and nature of the organic thickening agent.
This content in surfactant will generally range from 0.1 to 5% by weight, preferably from 0.2 to 2% by weight and even more preferably from 0.5 to 1% by weight.
The thickening agent (a) according to the invention can be used in any decontaminating gel whatever the type, i.e. whatever the active agent of decontamination (b) implemented in the decontaminating gel.
The thickening agent (a) can especially be used instead of the exclusively mineral thickening agent, implemented in any of the decontaminating gels of the prior art as described for example in documents FR-A-2 380 624; FR-A-2 656 949 and FR-A-2 695 839, or it can be implemented instead of the thickening agent that consists of the combination of a mineral thickening agent and an organic thickening agent described in document FR-A-2 746 328.
We have seen that the decontaminating gels have different natures depending on the active agent of decontamination (b) they contain; they are gels called alkaline, acid, reducing and oxidizing.
Thus, the decontaminating gel according to the invention can contain as an active agent of decontamination (b), an acid, preferably a mineral acid, chosen preferably among hydrochloride acid, nitric acid, sulfuric acid, phosphoric acid and mixtures thereof.
The acid is generally present in a concentration ranging from 1 to 10 mol/l, preferably from 3 to 10 mol/l.
Such a gel, called xe2x80x9cacid gelxe2x80x9d, is particularly adapted to eliminate the cold-fixed contamination on the ferretic steels.
In this type of acid gel, the thickening agent is preferably a polyacrylic acid, preferably high average molar mass by weight, i.e. higher or equal to 450,000, for example around 4,000,000.
In this type of gel, the thickening agent is generally present in a concentration of 3 to 12% by weight.
The decontaminating gel according to invention can also contain as an active agent of decontamination (b), a base, preferably a mineral base, chosen preferably among the soda, potasium and mixtures thereof.
The base is generally present in a concentration of 0.1 to 14 mol/l.
Such a gel called xe2x80x9calkaline gelxe2x80x9d has interesting degreasing properties and is particularly adapted to eliminate the non-fixed contamination on the stainless steels and ferritic acids.
In this type of alkaline gel, the thickening agent is preferably an acrylic acid acrylamide copolymer, for example the type TEXIPOL(copyright) 63-510.
A typical example of decontaminating gel, basic or alkaline according to the invention is consists in a solution that includes:
from 9 to 11% by weight of an acrylic acid acrylamide copolymer of an average molar mass by weight of 200,000 and contains 20% of acrylamide by weight;
from 1 mol/l to 12 mol/l of soda, preferably 3 mol/l;
Such a basic thixotropic gel according to the invention without any thickening mineral charge has the following properties:
a lifetime of about one week;
a time of synthesis of 2 days;
Theological properties allowing the spraying without sagging;
easy rinsing with water in low pressure.
It should be noted that this gel does not need any heating during its synthesis.
The decontaminating gel according to the invention can also contain as an active agent of decontamination (b) a reducing agent that can be for example the agent described in document FR-A-2 695 839, in which the reducing agent used is a reducing agent that includes a normal potential of oxide-reduction Eo less than xe2x88x92600 mV/ENH (standard hydrogen electrode) in a high base medium (pH 13).
Examples of such reducing agents include borohydrides, sulphites, hydrosulphites, sulfides, hydrophosphites, zinc, hydrazine and mixtures thereof.
When borohydrides, sulphites, hydrosulphites, sulfides, hydrophosphites are used, they are in a metallic salt form, for example, salts of alkaline metals such as sodium.
When the sodium borohydride is used as reducing agent, the pH of the colloidal solution is preferably greater than or equal to 14, so that the borohydride remains stable.
The reducing agents, like it is described in document FR-A-2 695 839, are generally associated with a mineral base such as NaOH or KOH, in a concentration generally ranging from 0.1 to 14 mol/l. The concentration in reducing agent generally ranges from 0.1 to 4,5 mol/l.
In such a reducing gel, the thickening agent is rather an acrylic-acrylamide acid copolymer, for example of the type TEXIPOL(copyright) 63-510.
Such a gel called xe2x80x9creducing gelxe2x80x9d is generally used in complement and alternately with an oxidizing gel such as the one described below.
Such a gel allows especially to weaken and to move the metallic layers of oxide superficially adherent which are heat-deposited on the surface of alloys, such as austenitic stainless steels, Inconel and Incoy that form the primary circuits of pressurized-water reactors, which are not sensitive to the action of oxidizing decontaminating gel.
A typical example of the reducing decontaminating gel according to the invention is consists in the solution, which includes:
from 9 to 11% by weight of copolymer acid acrylic-acrylamide of average mass molar by weight 200,000, and containing 20% by weight of acrylamide;
from 1 to 12 mol/l of soda, preferably 3 mol/l;
from 1 to 4 mol/l of NaBH4, preferably 3 mol/l;
A gel reducing agent according to invention has the following characteristics:
the lifetime of gel is one week;
the time of synthesis is 2 days;
the rheological properties allow the spraying without sagging;
the rinsing is done in water under low pressure;
This gel does not require any heating during for its synthesis.
The gel of decontamination according to the invention can still contain, as active agent of decontamination b) an oxidizing agent.
This oxidizing agent can be, for example, an oxidizing agent such as the one described in document FR-A-2 656 949 in which the oxidizing agent used is an oxidizing agent that must have a normal potential of oxidation-reduction of more than 1400 mV/ENH in a strong acid environment (pH less than 1), which means an oxidizing power superior to the permanganate""s.
CeIV, CoIII, AgII and their mixtures can be mentioned as example of oxidizing agents.
In fact, the potentials of oxidation-reduction pairs corresponding to these oxidizing agents have the following values:
The use of these powerful oxidizing agents is particularly suitable when the surface to decontaminate is a metallic surface, for example in noble alloy, such as non-oxidizable steels 304 and 316L, Inconel and Incolloy.
Furthermore, these oxidizing agents can also oxidize some colloidal oxides not very soluble such as the PuO2, in order to turn them into a soluble form such as PuO22+.
In the decontaminating gel of the invention, the oxidizing agent can also be used in its reduced form, CeIII, CoII, AgI can be used for example, on condition that a compound able to oxidize this reduced form is added to the gel, or on condition that the gel is associated to another gel or to another colloidal solution, which contains a compound able to oxidize this reduced form of the oxidizing agent.
The compound able to oxidize the reduced form of the oxidizing agent can be made up of a persulfate of alkaline metal.
The oxidizing agents, of which Cerium (IV) is preferred, are generally associated to a mineral basis, or for more stability to a mineral acid such as HCl, H3PO4, H2SO4 and preferably to HNO3 at a concentration of 1 to 10 mol/l, preferably of 2 to 10 mol/l, preferably of 2 to 3 mol/l, for example 2,88 mol/l, the concentration in an oxidizing agent being in general within 0,1 to 2 mol/l, preferably within 0,6 to 1,5 mol/l preferably again this concentration is of 1 mol/l. When using an oxidizing cation such as CeIV, AgII, or CoIII as oxidizing agent, it can be entered as one of the salts like nitrate, sulfate or another, but can also be electro generated.
The preferred oxidizing gels contain Cerium (IV) in the form of electro generated nitrate of Cerium (IV) Ce(NO3)4, or hexanitrato cerate of di-ammonium (NH4)2Ce(NO3)4, the latter being preferred because of the relative instability of the nitrate of cerium (IV) in a nitric concentrated environment.
The nitric acid stabilizes the cerium at a degree IV of oxidation, takes part in the corrosion and assures, among other things, the maintaining in solution of the corroded species, which means of oxo-nitrate coordination of metals of transition constitutive to metallic alloy.
Such gels contain the organic thickening agent, preferably the polyacrylic acid at a coordination generally of 2 to 12% as weight.
Preferably, in this type of gel, the thickening agent is a polyacrylic acid, preferably a polyacrylic acid of average molar mass of relatively elevated weight, for example 4,000,000, but TEXIPOL can also be used, for example TEXIPOL 63-510, already described above.
This type of gel can also comprise, in addition to said thickening agent, a tension-active agent or surfactant such as described above, preferably C6E2 or C12E4, at a coordination of 0,1 to 1,5% in weight.
A first typical example of decontaminating oxidizing gel according to the invention is made up of a solution comprising:
10 to 13% by weight of acrylic acidxe2x80x94acrylamide copolymer with weight-average molecular weight of 200,000 and containing 20% of acrylamide.
2 to 3 mol/l preferably 2.88 mol/l of HNO3;
0,1 to 2 mol/l of (NH4)2Ce(NO3)6.
Such an oxidizing gel has the following properties:
lifetime of one week;
synthesis time between 2 and 10 days;
rheological properties allow spraying without running;
corrosive power of 0.3 xcexcm/2 hours/kg/m2;
easy rinsing with water under low pressure.
A second typical example of the oxidizing decontamination gel according to the invention formed by a solution containing:
7 to 8% by weight of polyacrylic acid with a weight-average molecular weight of 450,000;
2 to 3 mol/l, preferably 2.88 mol/l of HNO3;
0.1 to 2 mol/l of (NH4)2Ce(NO3)6;
up to 1% by weight of the surfactants preferably C6E2 or C12E4.
Such oxidizing gel containing polyacrylic acid as thickening agent presents the following properties:
lifetime between 2 and 5 days;
synthesis time approximately 2 days;
theological properties allowing spraying without running;
corrosive power between 0.3 and 0.7 xcexcm/2 hours/kg/m2.
A third typical example of the oxidizing decontamination gel according to the invention is formed by a solution containing:
0.6 to 1.2 mol/l preferably 0.9 mol/l of (NH4)2Ce(NO3)6 or Ce(NO3)4,
2 to 3 mol/l, preferably 2.88 mol/l of HNO3,
3,0 to 4,5% by weight, preferably 3,7% by weight of an acid polyacrylic with weight-average molecular weight of 400 000.
The characteristics of this gel are as follows:
Rheology: the time for setting is 1s and the viscosity is 12000 mPa.s in 5sxe2x88x921.
Corrosion: 1,33 xcexcm/4 hours for 1 kg of gel per m2, that is, 0,3 xcexcm/h; 1,07 xcexcm for 1 hour of application at 40xc2x0 C.; 0,95 xcexcm for xc2xd hour of the application at 80xc2x0 C.
Lifetime of gel: approximately 24 hours.
Rinsing: very easy under water with low pressure.
The decontamination gels described above could be used in particular for the decontamination of metal surfaces as well as in the scope of periodic maintenance of existing installations, and of dismantling nuclear installations.
The gels according to the invention could be used for example for decontamination of tanks, of the fuel storage containment pool, glove boxes, etc.
Also, the invention has for a goal, a decontamination process of a metal surface, that consists of the application of the decontamination gel according to the invention on the surface to be decontaminated, keeping this gel on the surface during a sufficient period to carry out the decontamination, this period could be between 10 min to 24 hours, preferably from 30 min to 10 hours, and preferably still from 2 to 5 hours, and for example, the removal of the gel from the metal surface so treated by rinsing it or by mechanical action.
According to this important and very particular According to this particularly important aspect of the invention due to the excellent properties of behaviour of the gels according to the invention with temperature, the surface to be decontaminated could be a surface whose temperature is, even permanently, greater than or equal to 40xc2x0 C., for example, from 40xc2x0 C. to 80xc2x0 C.
The quantities of the gel deposited on the surface to be decontaminated are in general from 100 to 2,000 g/m2 preferably from 100 to 1000 g/m2, preferably still from 200 to 800 g/m2.
It is obvious that we could repeat the treatment many times each time by using the same gel or the gels of different types during different successive stages, each of these stages consisting of the application of a gel, maintaining the gel on the surface, and removal of the gel from the surface, for example by rinsing or mechanical action.
Likewise, the treatment could be repeated over the entire surface to be treated or over only one part of that presenting for example a complex form, or as a function of the surface activity (mRad/h) with certain specific points of the latter requiring intensive treatment.
Also, one or more rinsings of the decontaminated surfaces may be carried out, in particular before the first application of the gel, with the aid of water or with an aqueous solution, preferably under high pressure, in order to cleanse and/or to degrease the surface to be treated.
For example, the decontamination procedure could comprise the following successive stages as is described in the document FR-A-2 695 839:
1) Apply on the surface to be decontaminated a reducing decontamination gel according to the invention, maintain this gel on the surface during a period between 10 min to 5 hours and rinse the metal surface to remove this reducing gel, and
2) Apply on the surface so treated, an oxidizing gel in acid environment, maintain this gel on the surface for a period between 30 min to 5 hours and rinse the metal surface so treated to remove the oxidizing gel.
Or clearly the decontamination process could consist of the following stages:
a. spraying on the surface to be decontaminated a solution of sodium hydroxide for a period, for example of 30 minutes,
b. rinsing with water
c. application on the surface so treated of an oxidizing gel in acid environment and maintaining it on the surface for a period of 30 minutes to 5 hours, preferably for two hours
d. rinsing with water.
The contact time can vary between wide limits and depends also on the type of the active decontamination agent and of the type of the organic thickening agent.
As an example, for an acid oxidizing gel containing a polyacrylic acid or TEXIPOL 63-510 as organic thickener, the contact period is preferably from 30 minutes to 5 hours, and more preferably from two to five hours.
For a reducing gel, the contact time would be preferably from 10 to 5 hours.
The application of gel over the metal surface to be decontaminated can be carried out by the standard procedures, for example from the spraying with the gun, soaking and draining, packaging or even by means of a paintbrush. Preferably, the gel is applied for spraying/sputtering with the gun, for example under pressure (Airless compressor) at the injector from 10 to 200 kg/cm2 for example, from 10 to 160 kg/cm2, for example also from 50 to 100 kg/cm2.
The gel can be removed from the treated surface preferably by rinsing, it could also be removed by other means for example, mechanics or with a gas jet, for example with compressed air.
To carry out the rinsing, demineralized water or an aqueous solution is used in which the gel used can be dissolved or in which it can form a film that can be detached and washed away with water.
The rinsing could be done under pressure, which means with a pressure from 10 to 160 kg/cm2.
According to a particular interesting characteristic of the invention, from the fact that the gels according to the invention, comprising a uniquely organic thickening agent keep for a prolonged period, could go up to 48 hours and more, their gel texture, rinsing of the surface is much more easy, can be done at a low pressure for example 15 kg/cm2 or even without pressure and require a reduced quantity of demineralized water, for example less than 10 l/m2.
The number of treatments (or passes) of the rinsing during a decontamination operation is reduced since the gel according to the invention does not include the mineral charge.
Again, as a result of the invention, the quantity of generated effluents defined especially from the volume of the rinsing effluents is greatly reduced.
On the contrary, the gels of the prior art, in which the thickening agent is partially or totally mineral, and which includes for example only the silica, become after application, and in a relatively short time, dry and cracked, their rinsing is very difficult and demand a larger amount of water under high pressure. Because of this, large quantities of the liquid effluents are generated.
The rinsing effluents are then treated appropriately, for example they can be neutralized, for example by the sodium hydroxide in the case where an acid gel has been used.
The effluents are then generally subjected to a solid-liquid separation, for example by filtration with a cartridge filter to give on the one hand, the effluent liquid, and on the other hand the solid wastes whose quantity is extremely reduced, even none, because of the very low mineral charge the gels according to the invention that in fact, come only from the active decontamination agent.
In the most cases, the quantity of the mineral in the gel according to the invention is likewise so low that it allows the transfer of the rinsing effluents towards an evaporator without any prior treatment.
The decontaminating gels of the invention can be prepared simply, for example by adding to an aqueous solution of constituent b) that is, of the active decontamination agent, the thickening agent, a) exclusively organic. In the case of an oxidizing gel in which the active agent b) includes, besides, the oxidizing agent, a mineral acid chosen, for example, among HNO3, HCl, H3PO4, H2SO4, and their mixtures, preferably HNO3, it turns out that the following preparation process was particularly advantageous, especially in terms of the preparation time; first are mixed the thickening agent a) the solution of mineral acid under agitation and possibly, heating, to solubilize the polymer and obtain the viscous and homogeneous acid gel and next to said acid gel is added, under agitation, the oxidizing agent, such as (NH4)2Ce(NO3)6.
The gels according to invention generally have a very long storage period; however the chemical inertia of certain surfactants although good is limited in time, for example in the presence of an oxidizing agent like Ce (IV).
The great solubility of these surfactants induces a rapid homogenization during their incorporation in the gel. Their introduction in the solution should be done, preferably then a short time before the usage of the gels for optimal effectiveness.